Geology and mineral deposits in the Hillsboro mining district, Sierra County, New Mexico
— Pruthvid Geedipally, Virginia T. McLemore, and Nelia Dunbar

The Copper Flat porphyry copper deposit in the Hillsboro mining district in the Animas Mountains in central New Mexico is one of the older Laramide-age porphyry copper deposits in the Arizona-Sonora-New Mexico porphyry copper province. The Copper Flat porphyry copper deposit has measured and indicated reserves of 194 million short tons at 0.26% Cu, 0.008% Mo, 0.002 oz/short ton Au, and 0.05 oz/ short ton Ag and inferred reserves of 8 million short tons of ore at a reported grade of 0.23% Cu, 0.004% Mo, and 0.01 oz/short ton Ag. The Hillsboro district consists of Cretaceous andesites surrounded by Paleozoic sedimentary rocks and Quaternary alluvial fan deposits. A quartz monzonite stock (74.93±0.66 Ma, ⁴⁰Ar/³⁹Ar) hosting a mineralized breccia pipe is located in the center of the district and a series of latite dikes radiate outwards from this quartz monzonite porphyry. The quartz monzonite porphyry and the latite dikes are co-genetic. Base-metal replacement deposits occur near the porphyry deposit and are genetically related to porphyry deposit. The Copper Flat porphyry copper deposit consists of Cu, Au, Mo, and Ag hosted in disseminated sulfides and sulfide-bearing veins in the quartz monzonite and a central breccia pipe. Chemical analyses of surface samples of the porphyry copper deposit indicates that they are enriched in Cu and Mo and depleted in Ag, As, Pb, Zn, Ba, Bi, Mn, and V relative to the vein and replacement deposits. Propagating outward radially from the Copper Flat porphyry copper deposit are Laramide polymetallic veins hosted by the latite dikes. The veins vary tremendously in chemical composition, but are typically enriched in Au, Ag, Cu, As, Bi, Cd and depleted in Mo relative to the porphyry copper and carbonate-hosted replacement deposits and contain as much as 8,560 ppm As, 385 ppm Cd, 3,400 ppm Bi, and 130 ppm Te. Carbonate-hosted replacement deposits are found distal from the porphyry center of the district. Replacement deposits are enriched in Pb, Zn, Ba, V, and depleted in Au and Cu relative to the vein and porphyry-copper deposits and contain as much as 45 ppm As, 66 ppm Bi, and 93 ppm Cd in some samples. No discrete particles containing Au or Ag were identified using the electron microprobe, although electron microprobe studies show Au, Ag, and Mo are found in some minerals, but Te, Se, Cd, Bi, and As were not detected.